1. Field of the Invention
This invention relates in general to subsea wellhead equipment, and in particular to a casing or tubing hanger having a metal-to-metal seal.
2. Description of the Prior Art
A typical subsea wellhead assembly includes a wellhead housing mounted within a temporary guide base that is supported on the ocean floor. Large diameter conductor pipe is secured to the wellhead housing and extends downward into the each a short distance. A wellhead is mounted inside the wellhead housing and to a permanent guide base which mounts on top of the temporary guide base. Surface casing secured to the wellhead extends a few hundred feet down into the well. The top of the wellhead is connected to pressure equipment and risers that extend to a drilling vessel at the surface. As the well is drilled deeper, a first string of casing may be set to a certain depth. Subsequently, a second string of casing may be set.
In a typical installation, the casing hanger includes a casing hanger body which is secured to the upper end of the casing string. The body is supported on an annular shoulder in the wellhead. After cementing, a seal and locking means is lowered into an annular clearance between the casing hanger body and wellhead bore. The seal normally includes an elastomeric ring which is compressed by compression rings between the casing hanger body and the wellhead bore. The locking means includes a split ring and various wedges, which are actuated by rotation of a running tool to lock the elastomeric seal in compression and to lock the casing hanger in the wellhead. Wickers, which are small parallel grooves, may be located in the wellhead bore for engagement by the split ring.
While successful, elastomeric seals may not have as long of a life as a metal-to-metal seal, particularly if subjected to heat. Metal seals, and combinations of metal and rubber seals, are commercially available. Improvements, however, are desirable.